Add-ons are great for covering certain needs, such as additional BBM. However, what if you were able to switch out unlimited incoming calls for 400mb data, etc.?
A lot of the items built into the current plans were add-ons at one time or another, with the exclusion of data. There should be no reason why there couldn't something based on a flat plan and adding on. That's how it used to be. You had a base plan, and you add on from there.